1. Field of the Invention
The present invention relates to an ink jet recording apparatus adapted to discharge ink droplets to perform recording, and more particularly, to an ink jet recording apparatus of on-demand type which selectively discharges ink droplets. More specifically, the present invention is directed to an ink jet recording apparatus which performs recording on a recording medium held by a rotary drum.
2. Related Background Art
FIGS. 7 and 8 are perspective views showing background art of an ink jet recording apparatus.
FIG. 7 illustrates a background art example of a type which transports a recording medium in a planar state. The apparatus of FIG. 7 comprises a recording head 1 for discharging ink droplets to perform recording; a carriage 2 for supporting the recording head 1 which is movable in a direction perpendicular to a feeding direction of the recording medium; guide rails 3 for supporting and guiding the carriage 2; a motor 4 for driving the carriage 2; a pulley 5 directly coupled to the motor 4; a dependent pulley 6 arranged opposite to the pulley 5; a wire 7 wound around and stretched between the pulley 5 and the dependent pulley 6 in order to transmit a driving force generated by the motor 4 to the carriage 2; a recording medium 9 such as a sheet of paper; a sheet feeding motor 10 for transporting the recording medium 9; a cap 12 for protecting discharging nozzles of the recording head 1 on standby from being dried; a roller 55 for transporting the recording medium 9; an urging roller 56 for urging the recording medium 9 onto the roller 55 by an energizing means, not shown; and an empty discharge box 57 located between the cap 12 and the recording medium 9 for the recording head 1 to perform so-called empty discharging such as discharging of ink droplets for other purposes than recording and preparatory discharging. An arrow 13 indicates the moving direction of the carriage 2, and an arrow 14 the rotating direction of the roller 15.
When the apparatus as constructed above performs recording, the recording head 1 having the discharging nozzles thereof protected by the cap 12 is separated from the cap 12, supplied with a driving force generated by the motor 4 and transmitted through the wire 7 wound around the pulley 5 and the dependent pulley 6, scanned together with the carriage 2 toward the recording medium 9, moved by a predetermined length near the recording medium 9, and then returned in the reverse direction, i.e., toward the cap 12. In the meantime, the recording head 1 reciprocally moves in the direction indicated by the arrow 13 and discharges ink droplets on a predetermined position to perform recording.
The recording medium 9, in turn, is transported in the direction indicated by the arrow 14 by a predetermined length every time one scanning operation of the recording head 1 is completed by means of the sheet feeding motor 14 and the roller 15. These operations are repeated to perform recording. However, in relation to this recording, if a recording apparatus employs a so-called on-demand type recording head which selectively discharges ink from discharging nozzles thereof, a time interval until discharging ink may be so long, depending on images, that the viscosity of ink existing near the nozzles is increased which may hinder normal operation. For this reason, the empty discharge box 17 is provided to solve this problem by periodically discharging ink even in a non-recording period. The empty discharge box 17 is located typically between the cap 12 and the recording medium 9, that is, not on the recording medium 9 but in the vicinity thereof, such that empty discharging is performed when the recording head 1, by the scanning of the carriage 2, reaches in front of the empty discharge box 17. The cap 12 can be used as a substitute for this empty discharge box 17.
Next, description will be given of the background art of a recording apparatus of a type which winds a recording medium around a rotary drum, as shown in FIG. 8.
The apparatus of FIG. 8 comprises a recording head 1 for discharging ink droplets to perform recording; a carriage 2 for supporting and carrying the recording head 1; guide rails 3 for supporting and guiding the carriage 2; a motor 4 for driving the carriage 2; a pulley 5 directly coupled to the motor 4; a dependent pulley 6 arranged opposite to the pulley 5; a wire 7 wound around and stretched between the pulley 5 and the dependent pulley 6 in order to transmit a driving force generated by the motor 4 to the carriage 2; a recording medium 9 mainly consisting of a sheet of paper; a sheet feeding motor 10 for transporting the recording medium 9; a rotary drum 8 rotatably coupled to the sheet feeding motor 10 for supporting the recording medium 9; and a cap 12 for protecting discharging nozzles of the recording head 1 on standby from being drying. These constituents are the same as those shown in FIG. 7. An arrow 13 indicates the moving direction of the carriage 2, and an arrow 14 the rotating direction of the rotary drum 8, which are also similar to the arrows 13 and 14 in FIG. 7. When the apparatus shown in FIG. 8 performs recording, the recording head 1 having the discharging nozzles thereof protected by the cap 12 is separated from the cap 12, supplied with a driving force generated by the motor 4 and transmitted by the wire 7 wound around the pulley 5 and the dependent pulley 6, and scanned together with the carriage 2 toward the recording medium 9. The rotary drum 8 is rotated together with the recording medium 9 in the direction indicated by the arrow 14 at a predetermined peripheral velocity by the driving force of the motor 10, while the recording head 1 discharges ink droplets on a predetermined position to perform recording. The recording head 1, upon completing recording of one line, moves to the recording position of the next line together with the carriage 2 to perform recording on that line. These operations are repeated to perform recording of each line.
The above described background art, however, implies the following problems to be solved.
The background art shown in FIG. 7 may comprise four to seven recording heads 1 respectively for discharging different color ink which are aligned to perform color recording. Such color recording can be performed only in one of going and returning movements of the carriage 2. This is because a color tone is changed from line to line due to the order of color deposition if changing in the going and returning processes.
Thus, an operation time required for the non-recording going or returning movement is useless, which results in delaying a recording time. However, if a moving speed of the recording head during non-recording is increased, an electric power consumption and noise are increased, and a larger motor must be employed as a result.
The background art shown in FIG. 9 does not imply so many problems as described above since recording cannot be performed only at a position where the recording medium wound around the rotary drum does not exist, that is, for a relatively short period in which the recording head cartridge 40 is not opposed to the recording medium. Nevertheless, in this background art, since the cap 46 is the only place to which the recording head cartridge 40 can discharge ink not for printing purpose during a printing operation, the on-demand type recording head adapted to selectively discharge ink droplets may suffer from defective discharging due to the aforementioned increase in viscosity of ink or the like. Particularly, for obtaining smaller ink droplets ranging approximately from 5 to 15 pl (pico liter) in order to record a high quality image, in contrast with ordinary ink droplets ranging approximately from 30 to 60 pl for recording an image in a pixel density of 400 dots/inch, the dimension of a discharging orifice of the recording head 1 and a discharging energy tend to be smaller, whereby the above-mentioned problem due to the increase in viscosity of ink becomes more serious.